Update Match3D/image-generation docs & code

Adds/updates documentation, assets and implementation for Match3D and puzzle image generation workflows. Key changes: decision logs and pitfalls updated to prefer VectorEngine Gemini for Match3D material sheets and to require edits (multipart) for 1:1 container reference images; guidance added for when to use APIMart vs VectorEngine. .env.example clarified APIMart/Responses config. Many new public assets and PPT visuals added. Code changes across frontend and backend: updated shared contracts, server-rs match3d/puzzle/image-generation handlers, VectorEngine/OpenAI image generation clients, and multiple React components/tests to handle UI/background/container image signing, edits workflow, and puzzle UI background resolution. Added src/services/puzzle-runtime/puzzleUiBackgroundSource.ts and related test updates. Includes notes about multipart HTTP/1.1 requirement and test/verification commands in docs.

src/services/puzzle-runtime/puzzleUiBackgroundSource.ts

@@ -0,0 +1,20 @@
+type PuzzleUiBackgroundFields = {
+  uiBackgroundImageSrc?: string | null;
+  uiBackgroundImageObjectKey?: string | null;
+};
+
+export function resolvePuzzleUiBackgroundSource(
+  level: PuzzleUiBackgroundFields | null | undefined,
+) {
+  const imageSrc = level?.uiBackgroundImageSrc?.trim();
+  if (imageSrc) {
+    return imageSrc;
+  }
+
+  const objectKey = level?.uiBackgroundImageObjectKey?.trim().replace(/^\/+/u, '');
+  if (!objectKey) {
+    return null;
+  }
+
+  return `/${objectKey}`;
+}